For instance, mobile communication terminals such as mobile phone, and pager, etc. and portable game machine use a vibrational motor as a means for informing the user of receiving of signal, occurrence of events, etc.
FIG. 1 is an exploded perspective view showing a structure of a BLDC vibrational motor (10) in a prior art used widely, and FIG. 2 is a cross-sectional view of the BLDC vibrational motor (10). The conventional BLDC vibrational motor (10) has a rotor rotating in high speed during operation, a stator remaining stationary and supporting the rotation of the rotor, and a case receiving them.
The rotor is formed by combining a back yoke (16), a bearing (18), a weight (20), and a magnet (22) into a single body. In order to shield magnetic field of the magnet (22), the bearing (18) is pressed forcefully in the back yoke (16). Inside the back yoke (16) is attached the magnet (22). Outside or on the top of the magnet (22) is attached the weight (20) made of material having a high density such as tungsten, making the center of mass of the rotor eccentric.
The stator has a PCB (32), an operating IC (30), a coil (28), and a shaft (26). In order to increase rotational torque and an operational performance of starting of the rotor, the rotor needs to be stopped in a certain segment. For that, in a bracket (36) of the stator is installed a cogging plate (34) for generating cogging torque. At a center of the bracket (36) where the cogging plate (34) is installed is provided a through-hole. In the through-hole is inserted and fixed the shaft (26). On the PCB (32) having a shaft through-hole provided in the center are installed one or more coils (28) (preferably, 2˜4 coils) for generating electrical force and the operating IC (30). The stator is completed by inserting the PCB (32) into the shaft (26) and attaching on the inner top portion of the bracket (36) with glue.
Assembling of the rotor with respect to the stator is done by inserting a central bearing (18) of the rotor around the shaft (26) of the stator. By inserting washers (24) on top and bottom portions of the shaft (26) after assembling, the rotor is limited to rotate within a certain height range. A top case (12) for protecting components inside is assembled with the bracket (36). Here, in order to prevent friction noise during rotation, a sliding film or washer (14) is inserted inside the top case (12).
The operating IC (30) has a Hall sensor, and supplies a current to a corresponding coil (28) using signals generated by the Hall sensor. The PCB (32) attached to the bracket (36) is installed extended up to the outside of the motor, so as to make it easy to connect a power source externally. If the external power is supplied to the PCB (32), current is supplied to the coils (28) installed inside through the operating IC (30) with the Hall sensor embedded. Each of the coils (28) flowing current forms a magnetic field, and interacts with the magnetic field of the rotor magnet (22), generating attracting and repelling forces, and thus rotating the rotor. Here, due to the eccentric center of mass by the weight (20), the rotor shakes and rotates, thus generating vibrations.
Recent mobile phones adopt a touch screen, enabling thin smart phones. With smart phone's thickness getting smaller, the components are under pressure to be made smaller and slimmer accordingly. The vibrational motor is not an exception, and its thickness is required to be thinner but the vibrational power is required to be larger. In the case of conventional BLDC vibrational motor (10), it is unavoidable to reduce the volume of rotor for getting smaller and slimmer. Since the weight (20) is to be disposed outside or over the magnet (22), it is also unavoidable to reduce the size of the weight (20) as those of other components. As the volume of the weight (20) is reduced, the eccentricity amount in rotation is reduced, resulting in reducing the vibrational force accordingly. Like this, the conventional BLDC vibrational motor (10) has a structural flaw of the vibrational force reduced along with its reduced size and/or thickness.
Also, in order to provide a structure that the bearing (18) supports the back yoke (16) in the conventional BLDC vibrational motor (10), a step (19) is provided on an exterior side surface by a difference of thickness in a cylindrical body of the bearing (18), and a bearing support (17) is provided at a center of the yoke (16). The bearing support (17) is inserted forcibly around a cylindrical body of the thin portion forming the step (19) of the bearing, by which the yoke (16) can engage with the bearing.
Since the bearing support (17) takes up some of the thickness in a radial direction in the structure that the bearing support (17) is provided in the back yoke (16), there is a limit to reducing the inner diameter of the magnet (22), compared to a structure in which the bearing support (17) is not provided. If the inner diameter of the magnet (22) gets larger, there is a limit to reducing its exterior diameter. For the magnet (22) has to be lager in size than the minimum size for generating the required magnetic flux. If the stator's coil (28) gets off from a region straightly below the magnet (22) and is disposed toward a center of the vibrational motor (10), that is, a little closer to the shaft (26) in such a structure, the magnetic flux interlinkage between the coil (28) and the magnet (22) gets reduced, which results in decrease of the magnetic force. That is, the coil (28) must be disposed right below the magnet (22), maximizing the overlapped area between them (the effective area of the coil (28) per unit area of the magnet (22) must be maximized) and then generating a maximum vibrational force. However, the conventional vibrational motor (10) has a disadvantageous structure in that if the motor (10) is designed so, it becomes very hard to reduce its size and thickness. In other words, if considering the magnetic force between the magnet (22) and the coil (28), the dimension of the magnet (22) is hard to reduce or the magnet is hard to move toward the central axis of rotation. Since the coil (28) must be disposed according to the position of the magnet (22), it is hard to reduce the size of coil (28) and dispose it toward the central axis.
Also, it is hard to minimize the size of operating IC with the conventional chip package type. In the chip package type, the operating IC chip is attached to the bottom surface of lead frame by die bonding, a wire bonding is done between terminal portions of the lead frame and aluminum pad portion of the IC chip, and the final product is obtained after a series of assembling processes such as plastic molding, trimming, etc. In such IC package type, the overall package size cannot help getting larger than the size of the IC chip. Since conventional BLDC vibrational motor uses an operating IC chip manufactured by such a wire bonding package type, it hinders reducing the entire size of vibrational motor. An improvement is needed to the package type of operating IC chip.